In medical field, the anesthetic gases are commonly used to assist the treatment. With respect to different kinds of anesthetic gases, different concentrations must be adopted to meet the requirements of the clinical medicine, based on the age and the physical condition of the patient.
In the commonly used anesthetic instruments, the type of the anesthetic gas is generally manually input by the anesthetist, and the operation is extremely inconvenient. In recent years, attempts are being made to propose a practical method and apparatus for automatically identifying the type of anesthetic gas with high accuracy.
The measuring principle of the conventional method for identifying the type of anesthetic gases is generally based on the Non-dispersive Infrared (NDIR) technique. That is, by utilizing the characteristic that a certain gas exhibits absorption effect with respect to a certain wave band and passing infrared light waves of the wave band through gas sample to be detected, the type of the anesthetic gas is determined by solving a matrix equation utilizing the theory that the attenuated amount of the transmissive lights and the concentration of the gas to be detected satisfy the Beer-Lambert law.
However, the infrared absorption spectrums of commonly used anesthetic gases in clinic medicine (i.e. Desflurane, Isoflurane, Halothane, Sevoflurane, Enflurane) are concentrated within a wide range of 7-14 μm and overlapped with each other. Moreover, the Beer-Lambert law is applicable to monochromatic light. In practice, however, the infrared lights filtered by light filters from the infrared light source generally have a certain bandwidth.
Therefore, the above-mentioned matrix equation will become a non-linear matrix equation when the type of the gas to be detected is determined by utilizing the Beer-Lambert law, which makes the solving procedure extremely complicated.
Although the non-linear matrix equation can be simplified by performing a particular design with respect to the parameters of light filters (see U.S. Pat. Nos. 5,046,018, 5,231,591), the design and the manufacturing process for the light filters is of great complexity, resulting a rise in the cost of the apparatus for detecting anesthetic gases utilizing the light filters.